The present invention relates to overhead cranes which are used to support critical loads such as nuclear waste casks. More specifically, the present invention pertains to overhead cranes which are specifically designed such that failure of any load supporting element of the hoist assembly will be compensated for, whereby the crane will maintain control over the load despite failure of one of these elements. Overhead cranes of this type are generally shown by means of example by, U.S. Pat. No. 3,786,935, issued Jan. 22, 1974 to Vlazney et al.
Overhead cranes of the type referred to and which are commonly used in nuclear reactor facilities to transport critical loads such as casks containing nuclear fuel or nuclear waste material generally comprise a trolley supported on overhead tracks or rails. The trolley in turn supports a large power driven cylindrical drum. A wire rope is wound around the drum and supports a load bearing hoop by means of a sheave assembly. Since even extreme care in manufacturing cannot completely preclude the possibility of failure of any single component of the crane, design criteria dictate that the crane be constructed such that failure of any single element will be compensated for in such a manner that control over the load is maintained.
The overhead crane shown in the Vlazney et al. patent cited above is generally directed to means for providing an overhead crane which includes a single failure proof hoist. However, there are a plurality of drawbacks to the Vlazney mechanism. For example, the cited patent illustrates a pair of hooks intended to simultaneously support a load. The hooks are independent in that they each include their own support shafts, however, it will be noted that they are not, in fact, single failure proof since both of the hooks are supported by the same crosshead and failure of that crosshead will result in failure of both of the hooks. A further disadvantage of the Vlazney et al. device is that failure of one of the ropes will result in swinging motion of the load since the two ropes used to support the load are wound upon opposite ends of the drum.